Method of curing double base propellants

ABSTRACT

A PROCESS FOR CURING PLASTISOL PROPELLANT FORMULATIONS COMPRISING A CURING STEP AT ABOUT 120*-145*F. FOR ABOUT 2-6 HOURS FOLLOWED BY A CURING STEP AT FROM ABOUT 80*110*F. UNTIL CURING IS COMPLETED, AND THE, PRODUCT RESULTING FROM THIS PROCESS.

D. A. BERTA April 3, 1973 METHOD OF CURING DOUBLE BASE PROPELLANTS Filed Feb. 2, 1972 E 2% is; m m

n N Q 3 N I Q; S d M S m w IIIIWII I 15. M m 03 w 2 W .L m -Q M a United States Patent US. Cl. 149--97 Claims ABSTRACT OF THE DISCLOSURE A process for curing plastisol propellant formulations comprising a curing step at about 120-145 F. for about 2-6 hours followed by a curing step at from about 80.- 110 F. until curing is completed, and the product resulting from this process.

BACKGROUND OF THE INVENTION Plastisol compositions are well known in the propellant art and comprise finely divided particles of an organic dispersion or plastisol-grade polymer dispersed or suspended in a high boiling, organic liquid plasticizer which dissolves the solid polymer readily at elevated temperatures. Many of these plastisol systems are well known in the art and include polyvinyl chloride plastisols, cellulose ester plastisols, cellulose ether plastisols and nitrocellulose plastisols.

The use of plastisol compositions as slurry double base and composite modified double base propellants is well known in the art. By double base propellant is meant one in which the major ingredients are a high explosive such as nitroglycerin and a suitable high energy polymer such as nitrocellulose. When an oxidizer such as ammonium perchlorate is added to the double base propellant, a modified type of double base propellant termed composite modified double base propellant is produced. The slurry double base propellants provide certain advantages over base grain type of propellants.

Slurry propellants are more economical and lend themselves to formulation changes more readily than the base grain type. However, there are factors that limit slurry propellants for use in some particular applications. One important limiting factor is that double base slurry propellant formulations are known to shrink during cure. Usually this cure is accomplished by heating the slurry to an elevated temperature whereby the plasticizer (nitroglycerin, for example) solvates the polymer (nitrocellulose for example), and then cooling the mass to form the rigid gel. In some instances, this solvation is carried out at room temperature over an extended period of time. Very often this solvation is performed in the presence of a crosslinking agent for the nitrocellulose (such as any organic compound containing a plurality of isocyanate groups), whereby the reaction of the functional groups on the crosslinking agent with the OH groups of nitrocellulose, accompanies solvation, usually occurring after the nitrocellulose is solvated. Ordinarily the cure is conducted at aconstant temperature or a cure cycle consisting of several days ambient, cure followed by several days elevated cure. As mentioned, supra, undesirable shrinkage will result from this curing. This shrinkage results from: (1) the solvation of the polymer by the plasticizer, and (2) the shrinkage resulting from the cooling down from the cure temperature to ambient temperature following the elevated temperature cure. The first type of shrinkage is usually more critical as it results in a greater amount of stress than does the thermal shrinkage, because the shrinkage is occurring during hardening. This stress producing shrinkage during isothermal cure of the plastisol slurry double base propellants can lead to internal stress, cracks in the propellant, and failure at the case-bond interface, and is therefore a great problem.

SUMMARY OF THE INVENTION Accordingly, it is an obpject of the instant invention to produce a stress free propellant composition.

It is an additional object of the present invention to devise an improved method of curing plastisol or slurry propellant compositions.

It is still another object of this invention to minimize the total shrinkage resulting from the curing of plastisol (slurry) propellant compositions.

It is still another object of the instant invention to eliminate the stress producing shrinkage resulting from the curing of plastisol compositions.

These objects and others are accomplished in accord ance with the present invention which, generally described, comprises initially curing at a temperature from about 120 F. to 145 F. for about 2 to 6 hours, and then curing at about F. to F. until the curing is complete. By curing, it is meant, herein and throughout the rest of the specification, that the slurry system is heated in the presence of a crosslinking agent so that crosslinking as well as solvation occurs.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 and 2 show the results of a study of the shrinkage and hardness of crosslinked propellants as a function of time at various cure temperatures.

DESCRIPTION OF THE PREFERRED EMBODIMENT The objects of the instant invention are achieved by improving the standard curing procedure of prior art processes in one respect. Rather than cure at one temperature throughout or at an .ambient temperature for a while and then at an elevated temperature, unexpectedly superior results are attained if the plastisol composition is heated at about F. to 145 F. for about 2 to 6 hours, preferably at F. to 145 F. for about 4 hours, followed by heating at about 80 F. to about 110 F. until curing is completed, preferably at 90 F. to 110 F. While the length of time will vary depending upon the concentration of the various ingredients, the time lengths indicated are good indications of the procedure to be used for standard plastisol formulations. Plastisol nitrocellulose is the preferred polymer to be used. The p1astisol form of nitrocellulose rather than the fibrous form is to be used because it solvates at a much slower rate. Thus, the term plastisol is used in two contexts. On the one hand it describes a mixture of a polymer in an organic liquid plasticizer and on the other hand it describes a form of nitrocellulose-a form which is used in the plastisol mixture.

It was unexpectedly found that shrinkage rate increases rapidly with increased cure temperature but the rate of increase in hardness was not as temperature sensitive. This can be seen from an examination of FIGS. 1 and 2. From this unexpected data, it has been found that an initial cure at a higher temperature, such as F. would enable the system to quickly reach a certain level of shrinkage before the system became hard, or crosslinked, as indicated by FIG. 2. In this graph, the vertical axis is a measure of hardness, and crosslinking occurs when the curves level off. By having most of the shrinkage end prior to the cross linking, or while the system is still soft, the stress produced by shrinkage is much less than if shrinkage occurs after the system is already crosslinked, or hardened-which would occur if the cure takes place at 100 F., for example. The shrinkage in FIG. 1 generally depicts the shrinkage occurring due to solvation. If one heats the system at 140 F. for 2 to 6 hours, this shrinkage is completed and the only other shrinkage that can occur is the shrinkage occurring when it is cooled down to ambient temperature after the cure. This latter shrinkage is lessened if the system is brought down to about 90 F. to 110 F. after the initial cure and until the cure is completed and is then later cooled to ambient temperature. The shrinkage due to solvation will no longer occur at 90 F. to 110 F. because it was completed at 140 F. Thus, by initially curing at 140 F. one obtains, as the graphs indicate, less total shrinkage during the cure than an initial cure at 100 F. would produce, as well as shrinkage before the system is crosslinked or hardened, resulting in less stress producing shrinkage. By finishing the cure at 90 F. to 110 F one eliminates thermal shrinkage upon cooling.

The final heat treatment is conducted until the curing is complete. The completion of the cure is determined by measuring hardness and volume change. The termination of the cure can be determined by measuring the hardness. When the system reaches its peak hardness and the hardness levels off, curing is complete. This will generally be completed in 18 to 32 hours after the second step is started, preferably in about 24 hours.

As indicated by FIG. 1, a temperature of 140 F. would be more preferable than 120 F. While the 120 F. temperature achieves the primary object of maximum shrinkage before crosslinking, the secondary object of minimum total shrinkage during the heating period is favored by the 140 F. temperature. While cooling down from 140 F. produces more shrinkage than cooling down from 120 F. does, this advantage does not offset the greater shrinkage resulting from solvation during the respective heating steps.

While the instant invention is applicable to any plastisol propellant system, it is preferable to use one containing plastisol nitrocellulose as the polymer and a nitroglycerin containing plasticizer. Any conventional additive may be added. The propellant should preferably be of the composite modified type, containing anywhere from about 35 60 percent solids, including a standard oxidizer like ammonium perchlorate or aluminum, stabilizers, burning rate modifiers, blast suppressors and any other conventional composite modified double base propellant additives. The amount of 35-60 percent solids is exclusive of the nitrocellulose or any ingredient in the placticizer. The amount of nitrocellulose present will affect the shrinkage since shrinkage will usually increase with increasing plastisol concentration. Based on the mixture of plastisol nitrocellulose and liquid plasticizer in the absence of other solids, the amount of plastisol nitrocellulose present is generally from about 10 to 40 weight percent. The mixture of nitrocellulose and liquid plasticizer is present in the propellant in an amount of from about 40 to 65 percent by weight. The plasticizer should be nitroglycerin but can contain conventional additives such as 2-nitrodiphenylamine stabilizer and triacetin plasticizer. The crosslinking agent present can be any conventional crosslinking agent for nitrocellulose, such as isocyanate containing compounds containing two or more isocyanate groups. Exemplary crosslinking agents are toluene diisocyanate and PGA-TDI. PGA-TDI is the conventional, well known adduct of polyglycol adipate and tolylene diisocyanate. The amount of crosslinking agent present is also a standard variable in the art and is generally from 2 to 20 percent by weight of the composition, inclusive of minor amounts, not exceeding .01 percent by weight of the composition, of a conventional crosslinking catalyst such as dibutyltin diacetate. It is to be understood that the proportions recited herein are not critical to the invention, since the inventive concept lies in the modification of the curing procedures of well known formulations.

The following examples illustrate the method of per- 'forming the novel method of the instant invention.

EXAMPLE I A plastisol propellant containing 10 percent plastisol nitrocellulose, 40 percent plasticizer and 50 percent of 400-micron ammonium perchlorate is cured at 140 F. for 4 hours and then cured at F. for 24 hours, at which time the curing is complete. The stress producing shrinkage resulting from the cure is less than the shrinkage of a cure conducted at the 100 F. throughout, or at 140 F. throughout. The plasticizer comprises 75 percent NG, 24 percent triacetin and 1 percent NDPA (2-nitrodiphenylamine). The plastisol nitrocellulose comprises 8 percent of plastisol nitrocellulose (Du Pont PNC, X- 359), 1.997 percent of TDI, and .003 percent by dibutyltin diacetate.

EXAMPLE II The above procedure is carried out using aluminum as the oxidizer instead of the ammonium perchlorate, and N-92 as the plastisol nitrocellulose (Hercules plastisol nitrocellulose). Improved shrinkage results are obtained over a cure using the standard cure techniques.

Having fully described the invention, it is intended to be limited only by the lawful scope of the appended claims.

What is claimed is:

1. In a process of curing a plasticol propellant composition comprising a polymer, organic liquid plasticizer, and crosslinking agent, so as to solvate the polymer with the plasticizer and crosslink the polymer by reaction with the crosslinking agent, the improvement which comprises curing by means of a two-step process wherein the first step comprises heating at a temperature from about F. to 145 F. for a period of from about 2 to about 6 hours and the second step comprises heating at a temperature of from about 80 F. to about 110 F. until curing is complete, wherein the second step immediately follows the first.

2. The process of claim 1 wherein said first step is conducted at from about F. to about 145 F. and said second step is conducted at from about 90 F. to about 110 F.

3. The process of claim 2 wherein said first step is conducted at about F. and said second step is conducted at about 100 F.

4. The process of claim 1 wherein said polymer is plastisol nitrocellulose and said organic liquid plasticizer comprises nitroglycerin.

5. The process of claim 2 wherein said polymer is plastisol nitrocellulose and said organic liquid plasticizer comprises nitroglycerin.

6. The process of claim 3 wherein said polymer is plastisol nitrocellulose and said plasticizer comprises nitroglycerin.

7. The process of claim 6 wherein said first step is conducted for 4 hours.

8. The process of claim 1 wherein said plastisol propellant composition is selected from the group consisting of polyvinyl chloride plastisols, cellulose ester plastisols, cellulose ether pastisols and nitrocellulose plastisols.

9. The process of claim 2 wherein said plastisol propellant composition is selected from the group consisting of polyvinyl chloride plastisols, cellulose ester plastisols, cellulose ether plastisols and nitrocellulose plastisols.

10. The process of claim 3 wherein said plastisol propellant composition is selected from the group consisting of polyvinyl chloride plastisols, cellulose ester plastisols, cellulose ether plastisols and nitrocellulose plastisols.

References Cited UNITED STATES PATENTS 3,236,704 2/1966 Axelrod et a1 149-98 X 3,378,611 4/1968 Kincaid et a1 149-98 X 3,429,755 2/1969 Lampert 149-98 X STEPHEN J. LECHERT, JR., Primary Examiner US. Cl. X.R. 

